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Note 18 - Fair Value Measurements (Details Textual) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
AOCI, Debt Securities, Available-for-sale, Adjustment, after Tax, Total	$ 13,959,000	$ 1,713,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	1,642,000	2,091,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Acquisition		2,756,000
Fair Value, Inputs, Level 3 [Member]
AOCI, Debt Securities, Available-for-sale, Adjustment, after Tax, Total	$ 889,000	$ 324,000

X

- Definition

Amount of acquisition of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

No definition available.

+ Details

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Namespace Prefix:	cofs_
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Balance Type:	debit
Period Type:	duration

X

- Definition

Amount, after tax, of accumulated unrealized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=121641772&loc=d3e681-108580

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X

- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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